Directional hearing is the ability of a person to distinguish the direction in which a sound source is located. The ability to localize sounds is highly dependent on being able to perceive sounds in both ears. When sounds are inaudible in one ear, localization becomes very difficult. Reduced localization may lead to reduced safety, and difficulties in social functioning.
Further, listening with two ears enables a person to understand more when speech occurs in a noisy environment. This is because binaural hearing enhances speech understanding in noise because of several factors such as head diffraction, binaural squelch, and binaural redundancy. The head diffraction effect causes the signal-to-noise ratio (SNR) to be greater at one ear than the other when the noise and target speech arrives from different directions. Even when the same signal and noise reach both ears, the brain may combine both inputs to produce more salient central representations of the speech signal (binaural redundancy) than if only input from one ear is available. The brain may also make use of the inter-aural time and inter-aural level differences to at least partially reduce deleterious effects of noise (binaural squelch).
A person having severe to profound hearing loss in both ears but wears a cochlear implant in only one ear is an illustrative example where the person may experience considerable hearing deficits in localization and speech intelligibility. However, for unilateral cochlear implant users who have residual hearing in the non-implanted ear, it is possible to provide binaural hearing and take advantage of sound perception in both ears by fitting a hearing aid providing acoustic amplification to the ear with residual hearing. Thus, a bimodal hearing aid system is used where electrical stimulation on one ear is supplemented with acoustic amplification at the other ear having residual hearing.
These auditory units, i.e. cochlear implant and hearing aid providing acoustic amplification, are typically developed more or less independently without the possibility of their combined use being taken into account. These auditory units are usually also fitted separately, i.e. for fitting the auditory units to the recipient (user), different professionals separately and independently adjust parameters of each of the auditory unit at different clinics. These adjustments usually depend on features associated with the individual unit, hearing characteristics of individual ear of the recipient (user), along with different skills and judgment of the professionals. This commonly results in different loudness growth levels with respect to the two ears and distorted cue transmission. This may potentially lead to decreased wearing comfort and sub-optimal performance of the bimodal hearing aid system because ensuring a good binuaral loudness balance and optimal binaural cue transmission is basis for obtaining good localization ability and good speech recognition. Thus, the way these units are fitted are not optimal for their combined use in the hearing aid system.
Current bimodal fitting method has inherent challenges when trying to align loudness and ensuring undistorted binaural cue transmission. These challenges may include at least one or more of i) difference in loudness growth functions between electrically and acoustically stimulated hearing, ii) place mismatch, i.e. misalignment between respective mapping of acoustics to the auditory nerve in an acoustic hearing aid compared to the cochlear implant, iii) different electric and acoustic compression, iv) temporal asynchrony, i.e. total frequency-dependent delay of the electric path compared to the acoustic path because of processing in the CI and acoustic hearing aid before the auditory nerve is activated, v) brain plasticity implying that many of these psychoacoustic aspects changes over time after CI implantation and static fitting may not be optimal over a period of time, and vi) other individual differences such as changed acoustic loss on the ear with residual hearing loss and/or pathology on the ear with implanted cochlear implant.
Because it is very difficult to both measure and compensate for all these variations in a practical fitting situation, there exists a need to provide an efficient, easy to use and cost-effective solution that addresses at least some of the above-mentioned problems.